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                <ol class="chapter"><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">1.</strong> 类型系统</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../01_类型系统/Rust_Concept_Clarification_Deref_vs_AsRef_vs_Borrow_vs_Cow/Rust_Concept_Clarification_Deref_vs_AsRef_vs_Borrow_vs_Cow.html"><strong aria-hidden="true">1.1.</strong> Rust Concept Clarification Deref vs AsRef vs Borrow vs Cow</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust_Concept_Clarification_Deref_vs_AsRef_vs_Borrow_vs_Cow/Deref_AsRef_Borrow_Cow释义.html"><strong aria-hidden="true">1.2.</strong> Deref AsRef Borrow Cow 释义</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust的Borrow和AsRef：让你的代码用起来像呼吸一样自然/Rust的Borrow和AsRef：让你的代码用起来像呼吸一样自然.html"><strong aria-hidden="true">1.3.</strong> Rust的Borrow和AsRef：让你的代码用起来像呼吸一样自然</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust的Cow类型有什么用？详解Cow及其用途/Rust的Cow类型有什么用？详解Cow及其用途.html"><strong aria-hidden="true">1.4.</strong> Rust的Cow类型有什么用？详解Cow及其用途</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/判别Fn、FnMut、FnOnce的标准/判别Fn、FnMut、FnOnce的标准.html"><strong aria-hidden="true">1.5.</strong> 判别Fn、FnMut、FnOnce的标准</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/一行代码告诉你内部可变性的真相(UnsafeCell)/一行代码告诉你内部可变性的真相(UnsafeCell).html"><strong aria-hidden="true">1.6.</strong> 一行代码告诉你内部可变性的真相(UnsafeCell)</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Tour_of_Rust's_Standard_Library_Traits/Tour_of_Rust's_Standard_Library_Traits.html"><strong aria-hidden="true">1.7.</strong> Tour of Rust's Standard Library Traits</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/逆变、协变与子类型，以及Rust/逆变、协变与子类型，以及Rust.html"><strong aria-hidden="true">1.8.</strong> 逆变、协变与子类型，以及Rust</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust自引用结构、Pin与Unpin/Rust自引用结构、Pin与Unpin.html"><strong aria-hidden="true">1.9.</strong> Rust自引用结构、Pin与Unpin</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/为什么Rust需要Pin,Unpin/为什么Rust需要Pin,Unpin.html"><strong aria-hidden="true">1.10.</strong> 译：为什么 Rust 需要 Pin, Unpin ？</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/定海神针Pin和Unpin/定海神针Pin和Unpin.html"><strong aria-hidden="true">1.11.</strong> 译：定海神针 Pin 和 Unpin</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/sizedness-in-rust/sizedness-in-rust.html"><strong aria-hidden="true">1.12.</strong> Sizedness in Rust</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust生命周期集大成者PhantomData〈T〉/Rust生命周期集大成者PhantomData〈T〉.html"><strong aria-hidden="true">1.13.</strong> Rust生命周期集大成者 PhantomData&lt;T&gt;</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：用Rust做类型体操_Part_0.html"><strong aria-hidden="true">1.14.</strong> 数据库表达式执行的黑魔法：用Rust做类型体操 Part 0</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：GAT实现引用类型关联_Part_1.html"><strong aria-hidden="true">1.15.</strong> 数据库表达式执行的黑魔法：GAT实现引用类型关联 Part 1</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：用HRTB写bound_Part_2.html"><strong aria-hidden="true">1.16.</strong> 数据库表达式执行的黑魔法：用HRTB写bound Part 2</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：用Rust做类型体操之用宏展开重复代码_Part_3_&_4.html"><strong aria-hidden="true">1.17.</strong> 数据库表达式执行的黑魔法：用Rust做类型体操之用宏展开重复代码 Part 3 & 4</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：与Rust编译器斗智斗勇之表达式向量化_Part_5_&_6.html"><strong aria-hidden="true">1.18.</strong> 数据库表达式执行的黑魔法：与Rust编译器斗智斗勇之表达式向量化 Part 5 & 6</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：在Rust中用宏关联逻辑类型和实际类型_Part_7.html"><strong aria-hidden="true">1.19.</strong> 数据库表达式执行的黑魔法：在Rust中用宏关联逻辑类型和实际类型 Part 7</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">2.</strong> 生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../02_生命周期/Rust中的生命周期——从StrSplit实例说开去/Rust中的生命周期——从StrSplit实例说开去.html"><strong aria-hidden="true">2.1.</strong> Rust中的生命周期——从StrSplit实例说开去</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/与ChatGPT深度对话来学Rust生命周期/与ChatGPT深度对话来学Rust生命周期.html"><strong aria-hidden="true">2.2.</strong> 与ChatGPT深度对话来学Rust生命周期</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——early_bound与late_bound（1）/进击的Rust生命周期——early_bound与late_bound（1）.html"><strong aria-hidden="true">2.3.</strong> 进击的Rust生命周期——early_bound与late_bound（1）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——early_bound与late_bound（2）/进击的Rust生命周期——early_bound与late_bound（2）.html"><strong aria-hidden="true">2.4.</strong> 进击的Rust生命周期——early_bound与late_bound（2）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——一力降十会的MIR（1）/进击的Rust生命周期——一力降十会的MIR（1）.html"><strong aria-hidden="true">2.5.</strong> 进击的Rust生命周期——一力降十会的MIR（1）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——一力降十会的MIR（2）/进击的Rust生命周期——一力降十会的MIR（2）.html"><strong aria-hidden="true">2.6.</strong> 进击的Rust生命周期——一力降十会的MIR（2）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/Common_Rust_Lifetime_Misconceptions/Common_Rust_Lifetime_Misconceptions.html"><strong aria-hidden="true">2.7.</strong> Common Rust Lifetime Misconceptions</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/Rust生命周期常见误区/Rust生命周期常见误区.html"><strong aria-hidden="true">2.8.</strong> 译：Rust生命周期常见误区</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">3.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../05_无畏并发/简单写个Rust无锁队列/简单写个Rust无锁队列.html"><strong aria-hidden="true">3.1.</strong> 简单写个Rust无锁队列</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/进击的Rust多线程——混合自旋锁/进击的Rust多线程——混合自旋锁.html"><strong aria-hidden="true">3.2.</strong> 进击的Rust多线程——混合自旋锁</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/An_unsafe_tour_of_Rust's_Send_and_Sync/An_unsafe_tour_of_Rust's_Send_and_Sync.html"><strong aria-hidden="true">3.3.</strong> An unsafe tour of Rust's Send and Sync</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/进击的Rust多线程——Send与Sync/进击的Rust多线程——Send与Sync.html"><strong aria-hidden="true">3.4.</strong> 进击的Rust多线程——Send与Sync</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/进击的Rust多线程——离经叛道的PhantomData/进击的Rust多线程——离经叛道的PhantomData.html"><strong aria-hidden="true">3.5.</strong> 进击的Rust多线程——离经叛道的PhantomData</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/Rust_Async_Pin概念解析/Rust_Async_Pin概念解析.html"><strong aria-hidden="true">3.6.</strong> Rust Async: Pin概念解析</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/Rust和C++的并发库对比/Rust和C++的并发库对比.html"><strong aria-hidden="true">3.7.</strong> 译：Rust 和 C++ 的并发库对比</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/Rust原子类型和内存排序/Rust原子类型和内存排序.html"><strong aria-hidden="true">3.8.</strong> Rust原子类型和内存排序</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">4.</strong> 网络编程</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../06_网络编程/从编解码层面理解WebSocket_手写一个WebSocket/从编解码层面理解WebSocket_手写一个WebSocket.html"><strong aria-hidden="true">4.1.</strong> 从编解码层面理解WebSocket 手写一 个WebSocket</a></li><li class="chapter-item expanded "><a href="../../06_网络编程/透过Rust探索系统的本原：网络篇/透过Rust探索系统的本原：网络篇.html"><strong aria-hidden="true">4.2.</strong> 透过Rust探索系统的本原：网络篇</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">5.</strong> 轮子系列</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../07_轮子系列/700行Rust写一个内存分配器/700行Rust写一个内存分配器.html"><strong aria-hidden="true">5.1.</strong> 700行Rust写一个内存分配器</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/Rust：网络库的实现思路/Rust：网络库的实现思路.html"><strong aria-hidden="true">5.2.</strong> Rust：网络库的实现思路</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/Rust异步运行时基础部件/Rust异步运行时基础部件.html"><strong aria-hidden="true">5.3.</strong> Rust异步运行时基础部件</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/使用Rust+epoll编写异步IO框架/使用Rust+epoll编写异步IO框架（1）.html"><strong aria-hidden="true">5.4.</strong> 使用Rust+epoll编写异步IO框架（1）</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/使用Rust+epoll编写异步IO框架/使用Rust+epoll编写异步IO框架（2）.html"><strong aria-hidden="true">5.5.</strong> 使用Rust+epoll编写异步IO框架（2）</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/使用Rust+epoll编写异步IO框架/使用Rust+epoll编写异步IO框架（3）.html"><strong aria-hidden="true">5.6.</strong> 使用Rust+epoll编写异步IO框架（3）</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day1.html"><strong aria-hidden="true">5.7.</strong> 用rust从零开发一套web框架：day1</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day2.html"><strong aria-hidden="true">5.8.</strong> 用rust从零开发一套web框架：day2</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day3.html"><strong aria-hidden="true">5.9.</strong> 用rust从零开发一套web框架：day3</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day4.html"><strong aria-hidden="true">5.10.</strong> 用rust从零开发一套web框架：day4</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day5.html"><strong aria-hidden="true">5.11.</strong> 用rust从零开发一套web框架：day5</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">6.</strong> 奇技淫巧</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../08_奇技淫巧/Copy-On-Write是不是优化？/Copy-On-Write是不是优化？.html"><strong aria-hidden="true">6.1.</strong> 译：Copy-On-Write是不是优化？</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/揭秘神奇的Rust_Axum风格的函数实现/揭秘神奇的Rust_Axum风格的函数实现.html"><strong aria-hidden="true">6.2.</strong> 译：揭秘神奇的 Rust Axum 风格的函数实现</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/“变长参数”函数与回调/“变长参数”函数与回调.html"><strong aria-hidden="true">6.3.</strong> “变长参数”函数与回调</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/Rust字符串格式化的幕后：format_args!()/Rust字符串格式化的幕后：format_args!().html"><strong aria-hidden="true">6.4.</strong> 译：Rust字符串格式化的幕后：format_args!()</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/给Rust带来一点C++特产/给Rust带来一点C++特产.html"><strong aria-hidden="true">6.5.</strong> 给Rust带来一点C++特产</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/一步步实现_Rust_Bevy_ECS_的_System_简化版本/一步步实现_Rust_Bevy_ECS_的_System_简化版本.html"><strong aria-hidden="true">6.6.</strong> 一步步实现 Rust Bevy ECS 的 System 简化版本</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/Exploring_Design_Patterns_in_Rust_with_Algorithmic_Trading_Examples/Exploring_Design_Patterns_in_Rust_with_Algorithmic_Trading_Examples.html"><strong aria-hidden="true">6.7.</strong> Exploring Design Patterns in Rust with Algorithmic Trading Examples</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">7.</strong> 源码分析</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../09_源码分析/Rust并发：bytes源码分析/Rust并发：bytes源码分析.html" class="active"><strong aria-hidden="true">7.1.</strong> Rust并发：bytes源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust并发：标准库Arc源码分析/Rust并发：标准库Arc源码分析.html"><strong aria-hidden="true">7.2.</strong> Rust并发：标准库Arc源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust并发：标准库sync_Once源码分析/Rust并发：标准库sync_Once源码分析.html"><strong aria-hidden="true">7.3.</strong> Rust并发：标准库sync::Once源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust源码阅读：引用计数Rc/Rust源码阅读：引用计数Rc.html"><strong aria-hidden="true">7.4.</strong> Rust源码阅读：引用计数Rc</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust源码阅读：Cell、RefCell与内部可变性/Rust源码阅读：Cell、RefCell与内部可变性.html"><strong aria-hidden="true">7.5.</strong> Rust源码阅读： Cell、RefCell与内部可变性</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/关于_Rust_的_UnsafeCell、Cell_与_RefCell/关于_Rust_的_UnsafeCell、Cell_与_RefCell.html"><strong aria-hidden="true">7.6.</strong> 关于 Rust 的 UnsafeCell、Cell 与 RefCell</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust_Async_async-stream源码分析/Rust_Async_async-stream源码分析.html"><strong aria-hidden="true">7.7.</strong> Rust Async: async-stream源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/走进Tokio的异步世界/走进Tokio的异步世界.html"><strong aria-hidden="true">7.8.</strong> 走进 Tokio 的异步世界</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/tokio.rs_runtime的实现/tokio.rs_runtime的实现.html"><strong aria-hidden="true">7.9.</strong> tokio.rs runtime 的实现</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Tokio_internals/Tokio_internals.html"><strong aria-hidden="true">7.10.</strong> Tokio internals</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Tokio_internals/译文：Tokio內部机制.html"><strong aria-hidden="true">7.11.</strong> 译：Tokio 内部机制</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust_Axum_HTTP_框架的架构分析/Rust_Axum_HTTP_框架的架构分析.html"><strong aria-hidden="true">7.12.</strong> Rust Axum HTTP 框架的架构分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/安利一个Rust_Game_Engine：Bevy--ECS部分/安利一个Rust_Game_Engine：Bevy--ECS部分.html"><strong aria-hidden="true">7.13.</strong> 安利一个Rust Game Engine：Bevy--ECS部分</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Tokio_解析之任务调度/Tokio_解析之任务调度.html"><strong aria-hidden="true">7.14.</strong> Tokio 解析之任务调度</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">8.</strong> 生态观察</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../11_生态观察/Rust_web_frameworks_have_subpar_error_reporting/Rust_web_frameworks_have_subpar_error_reporting.html"><strong aria-hidden="true">8.1.</strong> Rust web frameworks have subpar error reporting</a></li><li class="chapter-item expanded "><a href="../../11_生态观察/SeaORM：要做Rust版本的ActiveRecord/SeaORM：要做Rust版本的ActiveRecord.html"><strong aria-hidden="true">8.2.</strong> SeaORM：要做Rust版本的ActiveRecord</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">9.</strong> 死灵终极</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_总述.html"><strong aria-hidden="true">9.1.</strong> 译：Learn Rust the Dangerous Way 总述</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_0.html"><strong aria-hidden="true">9.2.</strong> 译：Learn Rust the Dangerous Way 0</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_1.html"><strong aria-hidden="true">9.3.</strong> 译：Learn Rust the Dangerous Way 1</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_2.html"><strong aria-hidden="true">9.4.</strong> 译：Learn Rust the Dangerous Way 2</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_3.html"><strong aria-hidden="true">9.5.</strong> 译：Learn Rust the Dangerous Way 3</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_4.html"><strong aria-hidden="true">9.6.</strong> 译：Learn Rust the Dangerous Way 4</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_5.html"><strong aria-hidden="true">9.7.</strong> 译：Learn Rust the Dangerous Way 5</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测（一）.html"><strong aria-hidden="true">9.8.</strong> Unsafe Rust 随堂小测（一）</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测（二）.html"><strong aria-hidden="true">9.9.</strong> Unsafe Rust 随堂小测（二）</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测（三）.html"><strong aria-hidden="true">9.10.</strong> Unsafe Rust 随堂小测（三）</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测参考答案.html"><strong aria-hidden="true">9.11.</strong> Unsafe Rust 随堂小测参考答案</a></li></ol></li></ol>
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                        <h1 id="rust并发bytes源码分析"><a class="header" href="#rust并发bytes源码分析"><font size="5">Rust并发：bytes源码分析</font></a></h1>
<p>作者：<a href="https://www.zhihu.com/people/lai-zhi-chao-96">laizy</a></p>
<p>原载：<a href="https://zhuanlan.zhihu.com/p/109977513">https://zhuanlan.zhihu.com/p/109977513</a></p>
<p>bytes库Rust网络编程中使用比较频繁，其提供了和字节数组相关的一组抽象，能够极大避免内存拷贝。提供的主要组件如下：</p>
<ol>
<li>Bytes结构： 底层封装了一段连续的内存，提供了引用计数跟踪内存的生命周期，类似 <code>&amp;'static [u8]</code>，可供多个实例共享。</li>
<li>Buf/BufMut trait: 对逻辑上连续，但是底层表示可能不连续的Buffer抽象，分别类似 <code>&amp;[&amp;[u8]]</code>和 <code>&amp;[&amp;mut[u8]]</code>。</li>
<li>BytesMut结构：封装了一段可以修改但是后续可以生成Bytes共享出去的的连续内存。</li>
</ol>
<p>下面主要对Bytes进行分析，Buf/BufMut本身只是trait定义，一看就懂；为减少API，加快稳定，后面可能会移除掉BytesMut，所以有空再作分析。</p>
<h2 id="bytes内存布局"><a class="header" href="#bytes内存布局">Bytes内存布局</a></h2>
<p>下面是Bytes的大致内存布局图。底层的连续内存使用引用计数，每个实例持一个引用，以及记录本身所占有的区间。</p>
<p><img src="pict/001.webp" alt="" /></p>
<p>最简单的实现差不多是：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>struct Bytes {
    raw: Arc&lt;[u8]&gt;,
    data: usize,
    len: usize,
}
<span class="boring">}
</span></code></pre></pre>
<p>不过考虑到还有两类可优化的情况：</p>
<ol>
<li>底层的内存来自于 <code>&amp;'static [u8]</code>，那么本身就是可共享的，拷贝进Arc里，再进行引用计数，太浪费。</li>
<li>底层的内存来自 <code>Vec&lt;u8&gt;</code>或者 <code>Box&lt;[u8]&gt;</code>，且还没有发生共享只有一个Bytes实例的情况，如果这种情况能优化掉，那么Bytes的使用开销和Vec就几乎无差别了。</li>
</ol>
<p>对这两种情况的优化是Bytes的主要看点。</p>
<h2 id="bytes结构定义"><a class="header" href="#bytes结构定义">Bytes结构定义</a></h2>
<p>要考虑到底层内存的多样性，必然要使用多态。Bytes采用自定义虚表的方式实现多态。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct Bytes {
    // Bytes可访问的起始地址
    ptr: *const u8, 
    // Bytes可访问的数据长度
    len: usize,     
    // 底层的对象指针
    data: AtomicPtr&lt;()&gt;, 
    // 底层对象的虚表指针
    vtable: &amp;'static Vtable, 
}

pub(crate) struct Vtable {
    /// fn(data, ptr, len)
    pub clone: unsafe fn(&amp;AtomicPtr&lt;()&gt;, *const u8, usize) -&gt; Bytes,
    /// fn(data, ptr, len)
    pub drop: unsafe fn(&amp;mut AtomicPtr&lt;()&gt;, *const u8, usize),
}
<span class="boring">}
</span></code></pre></pre>
<h2 id="底层内存来自static-u8的情况"><a class="header" href="#底层内存来自static-u8的情况">底层内存来自<code>&amp;'static [u8]</code>的情况</a></h2>
<p>这种情况下，drop不需要做任何析构操作，clone操作也只是做简单的指针复制。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn from_static(bytes: &amp;'static [u8]) -&gt; Bytes {
    Bytes {
        ptr: bytes.as_ptr(),
        len: bytes.len(),
        //不需要记录引用计数
        data: AtomicPtr::new(ptr::null_mut()), 
        vtable: &amp;STATIC_VTABLE,
    }
}

const STATIC_VTABLE: Vtable = Vtable {
    clone: static_clone,
    drop: static_drop,
};

unsafe fn static_clone(_: &amp;AtomicPtr&lt;()&gt;, ptr: *const u8, len: usize) -&gt; Bytes {
    let slice = slice::from_raw_parts(ptr, len);
    Bytes::from_static(slice)
}

unsafe fn static_drop(_: &amp;mut AtomicPtr&lt;()&gt;, _: *const u8, _: usize) { }
<span class="boring">}
</span></code></pre></pre>
<h2 id="底层内存共享的情况"><a class="header" href="#底层内存共享的情况">底层内存共享的情况</a></h2>
<p>当存在共享时，数据和引用计数存在Shared结构中，定义如下：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>struct Shared {
    // 数据存放在Vec里，只用于析构，不直接访问
    _vec: Vec&lt;u8&gt;,
    ref_cnt: AtomicUsize, // 引用计数
}

static SHARED_VTABLE: Vtable = Vtable {
    clone: shared_clone,
    drop: shared_drop,
};
<span class="boring">}
</span></code></pre></pre>
<h2 id="clone的实现"><a class="header" href="#clone的实现">clone的实现</a></h2>
<p>读取Shared结构，并递增引用计数.</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>unsafe fn shared_clone(data: &amp;AtomicPtr&lt;()&gt;, ptr: *const u8, len: usize) -&gt; Bytes {
    let shared = data.load(Ordering::Acquire);
    shallow_clone_arc(shared as _, ptr, len)
}

unsafe fn shallow_clone_arc(shared: *mut Shared, ptr: *const u8, len: usize) -&gt; Bytes {
    let old_size = (*shared).ref_cnt.fetch_add(1, Ordering::Relaxed);

    if old_size &gt; usize::MAX &gt;&gt; 1 {
        crate::abort();
    }

    Bytes {
        ptr,
        len,
        data: AtomicPtr::new(shared as _),
        vtable: &amp;SHARED_VTABLE,
    }
}
<span class="boring">}
</span></code></pre></pre>
<h2 id="drop-的实现"><a class="header" href="#drop-的实现">drop 的实现</a></h2>
<p>递减引用计数，如果是最后一个引用，则进行析构。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>unsafe fn shared_drop(data: &amp;mut AtomicPtr&lt;()&gt;, _ptr: *const u8, _len: usize) {
    // 和clone不一样的是这里data是&amp;mut，不存在并发，所以直接读取。
    let shared = *data.get_mut();
    release_shared(shared as *mut Shared);
}

unsafe fn release_shared(ptr: *mut Shared) {
    // 这部分逻辑是从标准库Arc里的拷贝过来的。Release保证了drop前
    // 对Bytes的访问不会越过这条原子操作。下面的Acquire fence保证
    // 了Box::from_raw(ptr)以及后续的析构不会上移到这条原子操作。
    // 这样确保线程A的析构和线程B的访问建立跨线程同步关系。
    // 也可以直接采用fetch_sub(1, Ordering::AcquireRelease)，不
    // 过由于这是热点路径，大部分都满足if条件直接return了，因此理论
    // 上这样子会损失部分性能。
    if (*ptr).ref_cnt.fetch_sub(1, Ordering::Release) != 1 {
        return;
    }
    atomic::fence(Ordering::Acquire);
    Box::from_raw(ptr);
}
<span class="boring">}
</span></code></pre></pre>
<h2 id="底层采用vec不共享的情况"><a class="header" href="#底层采用vec不共享的情况">底层采用Vec不共享的情况</a></h2>
<p>这种情况是最复杂的，原因有二：</p>
<ol>
<li>在Bytes上调用clone方法，会使其变为共享状态，因此底层表示需要切换；</li>
<li>Bytes实现了Send和Sync，因此可以将&amp;Bytes发送到其他线程，导致clone方法可以并发调用。</li>
</ol>
<p>下面是根据Vec来构造Bytes的过程。为了区分在并发clone的时候，底层表示是不是已经切换为共享状态，采用的方式是将Bytes::data指针最低一位来表示，为0表示进入共享状态，为1表示为Vec状态。这种方式可行是因为共享状态的Shared结构至少是按usize对齐，指针最低位永远为0。不过Vec的数据指针是按照1字节对齐的，所以为进一步区分，引入了两个虚表： <code>PROMOTABLE_EVEN_VTABLE</code>和 <code>PROMOTABLE_ODD_VTABLE</code>。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>const KIND_ARC: usize = 0b0;
const KIND_VEC: usize = 0b1;
const KIND_MASK: usize = 0b1;

impl From&lt;Vec&lt;u8&gt;&gt; for Bytes {
    fn from(vec: Vec&lt;u8&gt;) -&gt; Bytes {
        // vec长度为空时按照静态共享的情况处理
        if vec.is_empty() {
            return Bytes::new();
        }

        // Vec转slice再拿到起始指针和长度
        let slice = vec.into_boxed_slice();
        let len = slice.len();
        let ptr = slice.as_ptr();
        drop(Box::into_raw(slice));

        // 按指针的奇偶性分别赋对应的虚表，同时保证data字段最低位为1。
        if ptr as usize &amp; 0x1 == 0 {
            let data = ptr as usize | KIND_VEC;
            Bytes {
                ptr,
                len,
                data: AtomicPtr::new(data as *mut _),
                vtable: &amp;PROMOTABLE_EVEN_VTABLE,
            }
        } else {
            Bytes {
                ptr,
                len,
                data: AtomicPtr::new(ptr as *mut _),
                vtable: &amp;PROMOTABLE_ODD_VTABLE,
            }
        }
    }
}
<span class="boring">}
</span></code></pre></pre>
<h2 id="clone的实现-1"><a class="header" href="#clone的实现-1">clone的实现</a></h2>
<p>先读取data，看看是否已经是共享状态了，如果是走共享的clone实现，如果不是走Vec的clone，奇偶性的实现唯一区别就是是否要把末尾的1移除掉。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>static PROMOTABLE_EVEN_VTABLE: Vtable = Vtable {
    clone: promotable_even_clone,
    drop: promotable_even_drop,
};

unsafe fn promotable_even_clone(data: &amp;AtomicPtr&lt;()&gt;, ptr: *const u8, len: usize)
   -&gt; Bytes {
    let shared = data.load(Ordering::Acquire);
    let kind = shared as usize &amp; KIND_MASK;

    if kind == KIND_ARC {
        shallow_clone_arc(shared as _, ptr, len)
    } else {
        debug_assert_eq!(kind, KIND_VEC);
        // 因为是原来的指针是偶数，所以这里把末尾的1去掉。
        let buf = (shared as usize &amp; !KIND_MASK) as *mut u8;
        shallow_clone_vec(data, shared, buf, ptr, len)
    }
}

unsafe fn shallow_clone_vec(atom: &amp;AtomicPtr&lt;()&gt;, ptr: *const (), buf: *mut u8, 
    offset: *const u8, len: usize) -&gt; Bytes {
    // 这里先尝试重构出原来的Vec，注：按rust目前的alias模型来说，
    // 这里会并发调用，多个线程可以同时构造出Vec实例来是有UB的，
    // 不过好像也想不到有更好的做法。
    let vec = rebuild_boxed_slice(buf, offset, len).into_vec();
    let shared = Box::new(Shared {
        _vec: vec,
        // 引用计数设为2，自己一个，clone出的一个
        ref_cnt: AtomicUsize::new(2),
    });

    let shared = Box::into_raw(shared);

    // shared必然是对齐的，因此最低bit一定为0
    debug_assert!(
        0 == (shared as usize &amp; KIND_MASK),
        &quot;internal: Box&lt;Shared&gt; should have an aligned pointer&quot;,
    );

    // 尝试原子地替换
    let actual = atom.compare_and_swap(ptr as _, shared as _, Ordering::AcqRel);

    if actual as usize == ptr as usize {
        // 替换成功，返回
        return Bytes {
            ptr: offset,
            len,
            data: AtomicPtr::new(shared as _),
            vtable: &amp;SHARED_VTABLE,
        };
    }

    // 其他线程抢先了，只好把构造的shared析构掉,但Vec是共享的，不能析构。
    let shared = Box::from_raw(shared);
    mem::forget(*shared);

    // 既然已经是共享状态了，就调用共享版本的clone。
    shallow_clone_arc(actual as _, offset, len)
}
<span class="boring">}
</span></code></pre></pre>
<h2 id="drop的实现"><a class="header" href="#drop的实现">drop的实现</a></h2>
<p>先读取data，看看是否已经是共享状态了，如果是走共享的drop实现，如果不是 则还原出Box&lt;[u8]&gt;并进行析构。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>unsafe fn promotable_even_drop(data: &amp;mut AtomicPtr&lt;()&gt;, ptr: *const u8, len: usize) {
    let shared = *data.get_mut();
    let kind = shared as usize &amp; KIND_MASK;

    if kind == KIND_ARC {
        release_shared(shared as *mut Shared);
    } else {
        debug_assert_eq!(kind, KIND_VEC);
        let buf = (shared as usize &amp; !KIND_MASK) as *mut u8;
        drop(rebuild_boxed_slice(buf, ptr, len));
    }
}
<span class="boring">}
</span></code></pre></pre>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>Bytes通过对&amp;'static [u8]和不共享时的情况进行了特殊优化，避免了内存拷贝和跨线程同步开销。同时其把可访问的区间内联进结构中，只把clone和drop的处理放置在虚函数里，使其在数据的访问上和 <code>&amp;[u8]</code>一样高效。</p>
<p>编辑于 2020-11-03 22:31</p>

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